There are several ways to book, and each has its proponents and critics. So, what follows is JHMO.

We've been on a couple of dozen cruises, and continue to use a travel agent. We've used the same TA for several years. We tend to value knowledge and excellent service more than saving what amounts to just a few dollars, if anything.

My suggestion if you choose to use a travel agent is to go to CLIA -- Cruise Lines International Association -- and search for a TA with CLIA certification. There's a search box on the lower right in which you enter your Zip code.